The ecteinascidins (herein abbreviated Et or Et's) are exceedingly potent antitumor agents isolated from the marine tunicate Ecteinascidia turbinata. In particular, Et's 729, 743 and 722 have demonstrated promising efficacy in vivo, including activity against P388 murine leukemia, B16 melanoma, Lewis lung carcinoma, and several human tumor xenograft models in mice. The antitumor activities of Et 729 and Et 743 have been evaluated by the NCI and recent experiments have shown that Et 729 gave 8 of 10 survivors 60 days following infection with B16 melanoma. In view of these impressive results, the search for additional ecteinascidin compounds continues.